1. Field of the Invention
The present invention relates to the field of energy, particularly storable energy, fuel cells, and proton exchange membrane fuel cells (e.g., PEMFC).
2. Background of the Art
Clean and highly efficient energy production has long been sought to solve environmental problems associated with the use of current energy sources, in particular the combustion of organic materials and especially the combustion of fossil fuels. Fuel cells, which convert the chemical energies stored in fuel directly into electrical energy, are expected to be a key enabling technology for the twenty-first century. Fuel cells have an enormous potential to provide reliable, clean energy and therefore are touted as ideal primary energy generators for remote locations and automobiles. (B. C. H. Steele and A. Heinzel, “Materials for Fuel-Cell Technologies” Nature, 414, 345-352 (2001); M. Winter and R. J. Brodd, “What Are Batteries, Fuel Cells, and Supercapacitors?” Chem. Rev., 104, 4245-4269 (2004).) Although fuel cells were used effectively in the Gemini space program in the early 1960s, they have not become a commercially viable industrial technology, largely owing to a lack of appropriate membrane materials. (M. Rikukawa and K. Sanui, “Proton-Conducting Polymer Electrolyte Membranes Based on Hydrocarbon Polymers” Prog. Polym. Sci., 25, 1463-1502 (2000).)
Among the types of fuel cells under active development, the proton exchange membrane fuel cell (PEMFC) is generally considered the most attractive power source for automotive use. In PEMFCs, the most important component is a proton exchange membrane (PEM), which separates the fuel from the oxidant but allows for proton (H−) transport from the anode to the cathode. For a proton-conducting material (typically a polymer electrolyte) to be used successfully as a PEM in PEMFCs, it must have (1) good chemical and electrochemical stability under fuel cell operating conditions, (2) good mechanical stability in both dry and hydrated states, (3) high proton conductivity, (4) zero electric conductivity, and (5) low production cost.
A PEM (Proton Exchange Membrane, also called Polymer Electrolyte Membrane) fuel cell uses a simple chemical reaction to combine hydrogen and oxygen into water, producing electric current in the process. For those interested in the chemistry, it works something like electrolysis of water in reverse order:
1. At the anode, hydrogen molecules give up electrons, forming protons (H+). This process is made possible by the platinum catalyst.
2. The proton exchange membrane allows protons to flow through, but not electrons. As a result, the protons flow directly through the proton exchange membrane to the cathode, while the electrons flow through an external circuit.
3. As they travel to the cathode through the external circuit, the electrons produce electrical current. This current can perform useful work by powering any electrical device (such as an electric motor or a light bulb).
4. At the cathode, the electrons and protons combine with oxygen to form water.
5. In a fuel cell, hydrogen's natural tendency to oxidize and form water produces electricity and useful work.
6. No pollution is produced and the only byproducts are water and heat.                Anode: 2H2--->4H++4e−        Cathode: 4e−+4H++O2--->2H2O        Overall: 2H2+O2--->2H2O        
Most membrane materials currently being tested in PEMFC demonstration units are based on sulfonated perfluoropolymers such as Nafion™ (Diagram 1[a]). These materials are essentially the same as those employed almost 30 years ago. Unfortunately, sulfonated perfluoropolymers have shortcomings that seriously limit their wide application in stationary or automobile power sources. These drawbacks include low proton conductivity at low humidity or high temperature (>100° C.), relatively low mechanical stability at high temperature, high cost, and high methanol permeability in direct methanol fuel cells. If membrane materials can be found that are capable of operating at high temperatures (˜120° C.), most of the shortcomings of current PEMs could be eliminated owing to the resulting benefits: (1) enhanced reaction kinetics, (2) simplified water management, (3) simplified thermal balance, (4) better heat recovery as steam that can increase the overall system efficiency of PEMFCs, and (5) reduced CO poisoning. Thus, the U.S. Department of Energy (DOE) and researchers around the world are making great efforts to develop alternative, low-cost, high-temperature, polymer-based electrolytes that have good chemical resistance, good mechanical stability, and sufficient proton conductivity (Table 1).
TABLE 1Technical Targets of Proton Exchange Membranes set bythe U.S. Department of EnergyiArea-Durability withOperatingspecificcycling (hours)PEM conductivitytemperatureresistanceCostAt ≦80° C.Characteristic(S/cm)(° C.)(Ohm-cm2)($/m2)At >80° C.Targets to be0.1 (at ≦120° C.)≦1200.02405000achieved by0.07 (at room temp)200020100.01 (at −20° C.)

Among currently known alternative membranes, BAM3G (Ballard Advanced Materials third-generation membrane) (Diagram 1[b]) from Ballard Advanced Materials (Burnaby, British Columbia)ii and sulfonated block copolymer of styrene-ethylene-butylene-styrene (SEBS) from Dais Analytic Corporation (Odessa, Fla.; Diagram 1[c]) have been semi-commercialized. BAM3G is considered the best commercially available membrane in terms of performance and chemical stability given the limitations of current practical fuel cell operating conditions. BAM3G is a partially fluorinated polystyrene-like electrolyte membrane in which C—F bonds are substituted at the benzylic position. The presence of an electron-withdrawing group (—CF—) at the benzylic position of the aromatic ring renders the sulfonic acid group as a stronger acid than typical aryl sulfonic acid. However, developing structurally related materials that may offer improved properties has proven difficult owing to presumed high cost and the limited availability of the trifluorostyrene monomers of BAM3G. In addition, the polystyrene-like polymer has flexible main-chain structure which may not be suitable for use in high-temperature (˜120° C.) fuel cell conditions.
Other types of high temperature polymer electrolyte membranes under investigation are chemically modified engineering polymers such as sulfonated poly(arylene ether sulfone) (Diagram 1[d]), sulfonated poly(arylene ether ether ketone) (Diagram 1[e]), sulfonated poly(phenylene) (Diagram 1[f]), and phosphoric acid-doped poly(benzimidazole) (Diagram 1[g]). These engineering polymers have been pursued as alternative PEM candidates because they can withstand the corrosive environments found in fuel cells. The sulfonated aromatic main-chain polymers, however, show sufficient proton conductivity only at high level of sulfonations where, unfortunately, they swell excessively on hydration and lose mechanical integrity above certain temperatures (60-80° C.). If this shortcoming cannot be overcome, it might prevent their use as high-temperature fuel cell membranes. Phosphoric acid-doped poly(benzimidazole) is known to show good proton conductivities at temperatures up to 200° C. The main disadvantage of the system is that the phosphoric acid molecules can diffuse out of the membrane at high temperatures because they are used in excess relative to basic sites of polymer.
Published U.S. Patent Application No. 20040224218A1 (Fan) describes a method and device for reducing or substantially eliminating methanol crossover from the anode to the cathode of a direct methanol fuel cell and for increasing catalyst efficiency in which a catalyst ink layer comprising an electron conductive and proton conductive binder material is applied either to the anode electrode or the electrolyte layer of the direct methanol fuel cell.
Published U.S. Patent Application No. 20030091886A1 (Tanioka) describes a polyelectrolyte comprising at least a styrenic polymer having a syndiotactic configuration and exhibiting an ion exchange capability, a polyelectrolyte membrane produced by forming the polyelectrolyte into a film, and a fuel cell using the polyelectrolyte membrane. The polyelectrolyte of the present invention is inexpensive and exhibits a good long-term stability, and is suitably used for fuel cells, production of common salt from sea water and recovery of acids from waste water.
European Patent No. 1179550 discloses the preparation of a polyelectrolyte membrane for fuel cells in which the polyelectrolyte comprises at least a styrenic polymer having a syndiotactic configuration (s-PS) as an essential component. The s-PS may or may not contain ion exchange groups therein. Accordingly, the polyelectrolyte is classified into two types, i.e., (1) those polyelectrolytes comprising an ion-exchange group containing thermoplastic resin other than s-PS, an ion-exchange group-free s-PS, and if required, the other ion-exchange group-free thermoplastic resin; and (2) those polyelectrolytes comprising a thermoplastic resin containing at least an ion-exchange group-containing s-PS, and if required, an ion-exchange group-free thermoplastic resin. As with the thermoplastic resins other than s-PS used in the polyelectrolytes (1) and (2), any suitable thermoplastic resins may be used without particular limitations. The weight-average molecular weight of the styrenic polymers is preferably 10,000 or higher, and more preferably 50,000 or higher. Of these styrenic polymers, syndiotactic polystyrene is especially preferred.